This research will investigate the relationship between the development of tolerance to the behavioral and physiological effects of smoked marijuana and self-administration of smoked marijuana. Chronic marijuana smokers will participate in single dose and repeated dose studies aimed at delineating the effects of repeatedly smoking marijuana over prolonged periods of time. Smoking topography, tolerance, and motivational changes will be measured in subjects living on a Clinical Research Unit for up to three weeks and allowed to smoke marijuana repeatedly. We will also assess Delta9THC blood levels and correlate these levels with subjective effects, task performance and marijuana self-administration. Of particular interest is assessment of behavioral tolerance and investigation of possible motivational changes associated with long term marijuana use. To this end, we have developed an operant procedure designed to separate the motoric and motivational effects of marijuana. We will use a cumulative dosing procedure for more efficient assessment of changes in sensitivity after marijuana. We will also collect smoking topography data with single marijuana dosing in order to demonstrate the sensitivity of the variables we are proposing to measure. Many people consume marijuana in the perhaps mistaken belief that it will have no adverse consequences the next morning. We will investigate whether such "hangover" effects do occur and, if so, their precise nature and the dosing conditions under which they develop. These studies should help in determining the conditions responsible for the development of tolerance to the behavioral actions of marijuana and whether such tolerance leads to an increase in smoking frequency. The studies of the motivational effects of marijuana should help to resolve the current contradictory data on the "amotivational syndrome." Finally, these studies should help to determine whether heavy marijuana smoking produces adverse effects on cognitive or motor function or mood on the following day. Such "hangover" effects may have important and as yet unrecognized public health implications.